To quantify components of energy expenditure on 24-hour basis in adolescents with HbSS; to determine changes in amount of work, work efficiency, and related EE increase caused by physical activities in adolescents with HbSS, and to determine level of habitual physical activity in free living adolescents with HbSS. Our future goals are to continue recruiting subjects outside the middle Tennessee area and recruit HbSS adolescents with a poor growth patterns.